This invention relates to coupled-cavity type traveling-wave tubes, and more particularly to coupled-cavity type traveling-wave tubes having improved sever attenuators coupled to the slow-wave circuit.
In coupled-cavity type traveling-wave tubes in general, the slow-wave circuit is divided into several sections, and the ends of the divided sections are terminated with the sever attenuators to prevent oscillation from occuring by reflection due to impedance mismatching between the slow-wave circuit and the input or output coupling stage and between the input or output coupling stage and the input or output circuit connected to the input or output coupling stage, thus permitting the tube to maintain stable amplifying operation.
The sever termination attenuators are expected to be capable of terminating the slow-wave circuit with good impedance matching, causing a minimum of gas up due to rise in the temperature of the attenuator body, and maintaining high heat-dissipating efficiency. To meet these requirements, prior art techniques have proposed a method as described in The Bell System Technical Journal, July 1963, pp. 1829-1861, in which the waveguide is led out from the severed end of the slow-wave circuit, and an attenuator structure comprising a ceramic material soaked with an attenuating substance is brazed to the inner wall of the waveguide. This technique, however, is impractible because the attenuator structure is very likely to crack due to heat applied during brazing in the process of making the tube or during the outgassing process or due to a rise in the temperature of the sever attenuators. This arises from the difference in thermal expansion coefficient between the material (e.g., copper) of the wall of the waveguide and the main component material (ceramics) of the attenuator structure. One solution of this problem has been to provide a structural improvement in which the thickness of the waveguide wall is reduced, the attenuator body is brazed thereto on the vacuum side, a ceramic plate for balancing thermal stress on the waveguide wall is brazed thereto on the nonvacuum side, and the thermal stress is absorbed by the thin waveguide wall by utilizing its characteristic of plastic deformation. In practice, however, thinning the waveguide wall to a thickness where the attenuator body and the ceramic plate can be free of stress is very likely to break the waveguide wall due to excess plastic deformation, resulting in a hole or crack in part of the outer wall which maintains a vacuum. This has made it extremely difficult to establish the normal operating reliability of the traveling-wave tube.